<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
  结果:<span></span>
</div>

<script>
  function f(x){
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");
    if(isNaN(i1.value||isNaN(i2.value))){
      s.innerText = "输入错误!"
      return;
    }
    switch(x){
      case 1 :
        s.innerText = parseFloat(i1.value)+parseFloat(i2.value);
        break;
      case 2 :
        s.innerText = i1.value-i2.value;
        break;
      case 3 :
        s.innerText = i1.value*i2.value;
        break;
      case 4 :
        s.innerText = i1.value/i2.value;
        break;
    }
  }
</script>

</body>
</html>